The present invention relates to a power converter using pulse width modulation (PWM) control and more particularly, relates to a method and apparatus for control of a PWM converter, which enables the performance of power regeneration operation from a direct current (DC) side to an alternating current (AC) side. One type of power converter is a PWM converter that is configured from switching devices of the self turn-off type such as gate turn-off (GTO) thyristors, giant transistors (GTR)--one kind of bipolar transistors--and the like, and these are widely used as direct current power sources for voltage-type inverters when AC motors are to be driven at variable speed.
FIG. 1 shows the general configuration of the main circuit of this type of PWM converter, and FIG. 2 shows the overall circuit configuration that includes a conventional control circuit. In FIG. 1, a PWM converter 1 comprises a bridge circuit in which a diode 1B is connected in reverse parallel with a switch circuit 1A of a self turn-off type. The converter 1 is connected to an AC power source 3 through an AC reacter, and is connected to a filter capacitor 4 at a DC power side. In FIG. 2, a load 5 is a load of the PWM converter and so, for example, can be thought of as being an inverter like the power reconverter. In the normal form of use, motors of air-conditioners and washing machines are further provided as further loads of the load 5. A voltage detection circuit 6 detects the DC voltage of the PWM converter 1. A voltage control circuit 7 compares the voltage reference V* and the DC voltage V, and outputs a current amplitude reference I* so that the difference between the two is reduced. A phase detection circuit 8 outputs sync signals (phase signals ) .theta..sub.PS in sync with the voltage of the AC power. A current reference circuit 9 outputs the three phase sine-wave AC current reference I.sub.R *, I.sub.S * and I.sub.T * that relate to the amplitude of the reference I* in sync with the AC power in response to the sync signals (phase signals) .theta..sub.PS and the current amplitude reference I*. A current control circuit 10R compares an R-phase feedback current I.sub.R detected by the current detector 12R with an R-phase current reference I.sub.R *, and outputs an R-phase voltage reference e.sub.R * so that the two become equal. Current control circuits 10S and 10T for an S-phase and a T-phase are similar. In sync with the phase signal .theta..sub.PS' a sawtooth wave generation circuit 13 outputs sawtooth wave carrier signals TR.sub.PS that have a frequency which is an integral number of times of the power frequency. A PWM control circuit 11 compares the sawtooth wave carrier signals TR.sub.PS with each of phase voltage reference e.sub.R * and e.sub.S * and e.sub.T * and outputs a switching reference SW of the self turn-off type device of the PWM converter 1.
In a PWM converter having the configuration described above, there is reversibility between the DC power and the AC power. When there is a power running operation where the load 5 consumes the DC power, the DC voltage V drops and a positive voltage difference occurs, and the voltage control circuit 7 outputs the current amplitude reference I* at a positive polarity so as to raise the DC voltage. Accordingly, the currents I.sub.R, I.sub.S and I.sub.T of each of the phases flow at the same phase as the AC power voltage V.sub.AC and power is supplied to the load side. On the other hand, when a regeneration operation, where the load 5 regenerates the DC power, is performed, the DC voltage V rises and a negative voltage difference occurs, and the voltage control circuit 7 outputs a current amplitude reference I* at a negative polarity so that the DC voltage drops. Accordingly, the reference values I.sub.R *, I.sub.S * and I.sub.T * of each of the phases become the opposite phase to the AC power voltage V.sub.AC and the actual I.sub.R, I.sub.s and I.sub.T of each of the phases also flow in the opposite phase, and DC power is regenerated to the AC circuit from the PWM converter 1.
FIG. 3A and FIG. 3B are the vector diagrams the power-running and regenerating operation, respectively. In the figures, V.sub.AC is an AC power voltage, I.sub.P is an AC current vector, V.sub.L is a reactance voltage vector of an AC reactor 2, and V.sub.IN is an input voltage vector of a PWM converter. As shown in FIG. 3A, when there is power-running, the AC power voltage V.sub.AC and I.sub.P are controlled to the same phase, and as shown in FIG. 3B, they are controlled to opposite phases when there is regeneration.
In a PWM converter having the configuration described above, it is necessary to increase the DC voltage of the PWM converter, in comparison with the AC power voltages and this increases the cost since it is necessary to use as a switching element a semiconductor element and a high-voltage proof element in order to proof a high-voltage. In particular, when there is a regeneration function, supplying power from the side of the load is a large problem since the I.sub.R drop and the reactance drop inside the apparatus require that the DC voltage be even higher than when there is power-running.